We have demonstrated in extracts of trophozoites of Entamoeba histolytica a cytotoxic enterotoxin which may contribute to virulence, tissue invasiveness, and diarrhea-producing characteristics of pathogenic strains of this protozoan parasite. It is proposed to increase production of axenic E. histolytica so that large quantities of organism will be available to characterize this amebic toxin and other toxins which may be found, as to their cytotoxic and enterotoxin effects in different assay systems; to localize sites of their synthesis and release with labeled specific antitoxin; to localize sites of their actions in tissues; and to further purify and biochemically define their structure and mechanism(s) of action.